1. Field of the Invention
The invention relates generally to carrier frequency recovery systems and more particularly to a carrier frequency estimator for estimating the carrier frequency of a QAM signal.
2. Description of the Prior Art
Modern communication signals carry information in the form of modulation symbols, where each of the modulation symbols represents a state for one or more bits of data. Each modulation symbol state has a particular in-phase (I) amplitude and a particular quadrature phase (Q) amplitude. When observed in a quadrature IQ plane, the I amplitudes and the Q amplitudes of the modulation symbols form a pattern or format. Many IQ formats have been used and proposed including formats termed 16 quadrature amplitude modulation (QAM), 64 QAM, 256 QAM, 1024 QAM, and the like.
FIG. 1 is an IQ diagram of the modulation symbol states for the 16 QAM format. The IQ diagram has an I axis and a Q axis. The 16 QAM format has sixteen modulation symbol states having relative I component amplitudes of -3, -1, 1, and 3 and relative Q component amplitudes of 3, 1, -1, and -3. Each modulation symbol state has a total amplitude or magnitude that is a square root of the sum of the squares of its I component amplitude and its Q component amplitude; and a phase that is an arc tangent of its I component amplitude divided by its Q component amplitude. A signal receiver uses locally generated signals for tracking (recovering) the carrier frequency and phase of an incoming communication signal in order to demodulate and decode the modulation symbols into data bits. When the locally generated frequencies do not match or have a frequency offset from the incoming frequency, the IQ diagram of FIG. 1 rotates at the rate of the frequency offset. This rotation causes the modulation symbols to change phase but not magnitude. A similar IQ diagram may be drawn for any format using modulation symbol states that can be described in terms of an I amplitude level and a Q amplitude level.
Quadrature amplitude modulation formats such as 16 QAM and the like have the benefits of being relatively easy to generate at the transmitting end of a communication signal link and relatively easy to decode into data bits at the receiving end. However, the circuitry for recovering the carrier frequency and phase is relatively complex and expensive because the communications signals for such formats do not have significant energy at the carrier frequency and therefore traditional simple phase lock loops are of no use. Typically, the complexity and expense increase where there is a relatively large frequency ambiguity over which a carrier recovery phase lock loop system is required to operate. Many specialized carrier recovery systems have been developed or proposed. However, all such systems have limitations and so there continues to be a need for improvement.